1. Field of the Invention
The present invention relates to a noise insulating wall structure, and more specifically relates to a noise insulating wall structure which is able to exhibit a noise insulating effect while maintaining gas permeability, and is particularly suitable for an under cover of an automobile.
2. Description of Related Art
Conventionally, as a noise insulating wall structure applied to an automobile, there is an automobile 1 shown in FIGS. 1(a), and 1(b). In the automobile 1, an under cover 5 is attached to a lower portion of the engine room. The under cover 5 functions to enhance the aerodynamic properties of the lower portion of the automobile 1, protect parts in the engine room 3 from impinging pebbles or the like, as well as a function of a noise insulating wall to attenuate noise producted by the automobile. The larger the area of the under cover 5 is, the more effect of the noise insulating wall has.
However, the larger the area of the under cover 5 is made, the more the lower portion of the engine room 3 is sealed, thereby increasing the temperature in the engine room 3. Consequently, the inside of the engine room 3 reaches a high temperature which affects durability of parts. Thus, in the design of the under cover 5 of the engine room 3, not only an aspect of noise control but also that of thermal control must be considered.
The present applicant had already filed an application relating to a noise insulating wall 7 shown in FIGS. 2 and 3 (see Japanese Application No. Hei-5-322041). This noise insulating wall 7 comprises two spaced noise insulating plates 9a and 9b which face each other. By providing hole portions 11a, 11b, 13a and 13b, a cylindrical portion 15 and an extension portion 23 in the noise insulating plates 9a and 9b and the spaced defined therebetween two or more types of vibration systems consisting of an air box and an air spring are formed, whereby transmission sound waves from the respective vibration systems interfere with one another and cancel each other. As a results, the noise insulating effect can be obtained.
Specifically, as shown in FIG. 3, the hole portions 11a and 11b which form a part of the plurality of hole portions 11a, 11b, 13a and 13b, communicate with each other by means of a straight tubular cylinder 15 having substantially the same internal cross-section as that of the respective hole portions 11a and 11b. Accordingly, a continuous hole portion 12 is formed by the hole portions 11a and 11b and the cylinder 15, which extends from the one noise insulating plate to the other. Air 17 in the continuous hole portion 12 acts as an air mass. The continuous hole portions 12 each form a vibration system 19 with one degree of freedom for the mass which receives sound pressure and applies an external force which periodically changes and is thus vibrated.
Further, in the hole portions 13a and 13b which are not communicated with each other by the cylinders 15, cylindrically shaped wall portions 26 protrude inwardly from the opening edge portions of the hole portions 13a and 13b on the opposite sides of the noise insulating plates 9a and 9b to form extensions 23. Thus, extended hole portions 14 and 14 facing each other is formed so as to be longer than the plate thickness of the noise insulating plates 9a and 9b. Further, spaces 16 are formed between the noise insulating plates 9a and 9b and around the extension 23. Air in the extended hole portions 14 and 14 acts as air masses and the air layer in the space 16 acts as an air spring, thereby forming a vibration system 21 with two degrees of freedom.
The above-mentioned vibration system 21 with one degree of freedom has no resonance frequency. An incident wave and a transmitted wave always have the same phase. On the contrary, the vibration system 19 with two degrees of freedom which is formed in the extended hole portions 14 and 14 has only one resonance vibration. In frequency bands which have one or more resonance frequencies, the phase of the incident wave and the phase of the transmitted wave exhibit an antiphase with respect to each other. Therefore, in a frequency band having one or more resonance frequencies in a the vibration system 21 with two degrees of freedom, the wave which passes through the continuous hole portion 12 and the wave which passes through the extended hole portions 14 and the space portion 16 exhibit an antiphase with respect to each other and mutually cancel each other, thereby obtaining the noise insulation effect.
In this case, the mass of the air mass which is defined in the hole portions 13a and 13b can be increased by changing the dimensions of the extensions 23 around the hole portions 13a and 13b. Thus, the resonance frequency for the frequency system 21 with two degrees of freedom can be decreased, and a noise insulating effect over a wider frequency band can be obtained.
In this noise insulating wall, the hole portions 11a, 11b, 13a and 13b are provided for noise insulating plates 29a and 29b. Accordingly, gas permeability can be ensured hot air within the engine room can easily be exhausted. Therefore, according to the thus constructed noise insulating wall, the under cover 5 with both characteristics of gas permeability and noise control can be obtained.
However, when a case where the noise insulating wall structure was actually used as the under cover 5 for an automobile is considered, water 25 can accumulate between the noise insulating plates 9a and 9b to the height of the extension 23 in which the noise insulating plate 9b on the lower side is protruding, by water splashes or the like while driving on a road with puddles, as shown in FIG. 3. In such a case, since the volume of the air layer between the noise insulating plates 9a and 9b is reduced, the air spring constant is increased and the resonance frequency of the vibration system 21 with two degrees of freedom is also increased. When the resonance frequency of the vibration system 21 with two freedom degrees is increased, the frequency band in which the noise insulating effect is narrowed. Accordingly, the noise insulating performance is deteriorated.
The deterioration of the noise insulating performance will be described with reference to FIG. 4. FIG. 4 shows a relationship between each frequency (Hz) for noise insulating walls in various cases, and the transmission loss TL (dB). In FIG. 4, a curve A shows calculated values of transmission loss TL in a case where there is no water between the noise insulating plates 29a and 29b (hereinafter referred to as a case of "no water"). On the other hand, a curve B shows calculated values of transmission loss TL in a case where there is water between the noise insulating plates 29a and 29b (hereinafter referred to as a case of "water existing"). Large transmission loss TL in these curves A and B shows that noises which are transmitted through the noise insulating wall are a little and that the noise insulating effect is large.
Further, a point C in the curve A, where the transmission loss TL is rapidly decreased denotes a resonance frequency for a vibration system 21 with two degrees of freedom in a case of no water, and a point D in the curve B, where the transmission loss TL is rapidly decreased denotes a resonance frequency for a vibration system 21 with two freedom degrees in a case of water existing. Further, a point E in the curves A and B, where the transmission loss TL is rapidly decreased at a frequency higher than the resonance frequency denotes a cavity resonance frequency for the cylinder of the vibration system 19 with one freedom degree, where air acts as the air mass. And, the region ranging from the point C of the resonance frequency for a vibration system 21 with two freedom degrees in a case of no water or the point D of the resonance frequency in a case of water existing, to the point E of the cavity resonance frequency forms a noise insulating region.
When water is collected between the noise insulating plates 29a and 29b, the resonance frequency for the vibration system with two freedom degrees is increased from the point C to the point D, as shown in FIG. 4.
Accordingly, the noise insulating region is changed from between C-E to between D-E, and is narrowed, that is, the noise insulating performance is deteriorated by the decreased region F.
In order to prevent this deterioration it has been proposed to provide the noise insulating plate 9b with a drain hole. However, the control of a mass spring system of both the vibration system 19 with one degree of freedom and the vibration system 21 with two degrees of freedom may become difficult to obtain a sufficient noise insulating performance.
This will be described by use of FIG. 5. FIG. 5 shows a relationship between each frequency (Hz) for noise insulating walls in various cases, and the transmission loss TL (dB). In FIG. 5, a curve G shows calculated values of transmission loss TL in a case where there is no hole in the noise insulating plates 9b, and a curve H shows experimental values of transmission loss TL, in a case where there is no hole in the noise insulating plates 9b. A curve K shows experimental values of transmission loss TL, in a case where there is a hole in the noise insulating plates 9b. In this connection, points L, M and N denote resonance frequencies for the vibration system with two freedom degrees, and a point 0 denotes a resonance frequency for the vibration system with one freedom degree. In this case, a region ranging from the resonance frequency L for the vibration system 21 with two freedom degrees to the resonance frequency 0 for the vibration system 19 with one freedom degree is a noise insulating region.
As shown in FIG. 5, when the curve H is compared with the curve K in a region P between the resonance frequencies L-O, in a case of the curve K, that is, a case where a hole is provided in the noise insulating plate 9b, the transmission loss TL for the vibration system with two degrees is remarkably smaller than in a case of the curve H. Therefore, when a hole is provided in the noise insulating plate 9b, it is necessary to control a vibration system again so that the transmission loss TL is not decreased, and but the control or the like of such vibration system would become difficult.